Quaternary Carboxylate Compositions for Extracting C1 to C4 Carboxylic Acids from Aqueous Streams

ABSTRACT

This invention relates to solvents for extracting C 1  to C 4  carboxylic acids from aqueous streams. More specifically, the extraction solvents include one or more salts composed of a tetraalkylphosphonium cation and a carboxylate anion. The extraction solvents may further include one or more non-ionic liquid organic solvents as an enhancer. The extraction solvents are useful for extracting aqueous mixtures containing one or more lower carboxylic acids, such as monocarboxylic acids, alkoxycarboxylic acids, and halogen-containing carboxylic acids.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No. 62/094,238 filed on Dec. 19, 2014 under 35 U.S.C. §119(e)(1); the entire content of the provisional application is hereby incorporated by reference.

FIELD OF THE INVENTION

The invention generally relates to solvents for extracting C₁ to C₄ carboxylic acids from aqueous streams, compositions containing the same, and processes for separating the acids from water.

BACKGROUND OF THE INVENTION

The recovery of C₁ to C₄ carboxylic acids (hereinafter “lower acids”) from aqueous streams is a common industrial problem arising from a variety of reaction and processing steps. Simple distillation of wet acid streams to recover glacial acids is hampered by unfavorable vapor-liquid equilibrium (VLE) and high energy costs with all C₁ to C₄ carboxylic acids. Examples of unfavorable VLE include the formic acid-water maximum-boiling homogeneous azeotrope, the acetic acid-water VLE “pinch” (a region of low relative volatility), and the minimum-boiling homogeneous azeotropes with water and all C₃-C₄ carboxylic acids.

Various approaches have been suggested in the art to address the problem of lower acid recovery from wet acid feeds. For example, one approach subjects an aqueous lower acid solution to azeotropic distillation together with an entraining component capable of forming a heterogeneous minimum-boiling azeotrope with water, so that the azeotrope boils at a temperature substantially lower than pure water, the pure lower acid, and any acid-water azeotrope. An extraction step often precedes the azeotropic distillation. The extraction step partitions the carboxylic acid into a water-immiscible solvent (which is often the same as the azeotropic entrainer) in order to remove the bulk of the water from the recovered acid. Many examples of azeotropic distillation, extraction, and combinations thereof using conventional organic solvents have been proposed in the art. These include U.S. Pat. Nos. 1,839,894; 1,860,512; 1,861,841; 1,917,391; 2,028,800; 2,050,234; 2,063,940; 2,076,184; 2,123,348; 2,157,143; 2,184,563; 2,199,983; 2,204,616; 2,269,163; 2,275,834; 2,275,862; 2,275,867; 2,317,758; 2,333,756; 2,359,154; 2,384,374; 2,395,010; 2,537,658; 2,567,244; 2,854,385; 3,052,610; and 5,662,780, and Eaglesfield et al., “Recovery of Acetic Acid from Dilute Aqueous Solutions by Liquid-Liquid Extraction—Part 1,” The Industrial Chemist, Vol. 29, pp. 147-151 (1953).

Several solvent characteristics determine the capital and energy costs of extraction-distillation processes for the extractive recovery of lower acids from wet acid feeds. The solvent for the extraction process is immiscible with water and meets two criteria:

-   -   a) The solvent shows some selectivity between extraction of the         carboxylic acid and water, i.e., the ratio of carboxylic acid to         water in the extraction solvent after extraction is         substantially larger than in the wet acid feed stream. This         factor can be quantified as the weight ratio of water to acid in         the extract stream as defined in more detail below.     -   b) The solvent shows sufficient affinity and capacity for the         lower carboxylic acid.         These characteristics are quantifiable from experimentally         determined equilibrium partition coefficients as defined in more         detail below.

The equilibrium partition coefficient (also used interchangeably with the term “partition coefficient”) for component A (the lower carboxylic acid) is defined as follows:

${P(A)} = \frac{{weight}\mspace{14mu} {percent}{\mspace{11mu} \;}A\mspace{14mu} {in}\mspace{14mu} {solvent}\mspace{14mu} {phase}}{{weight}\mspace{14mu} {percent}\mspace{14mu} A\mspace{14mu} {in}\mspace{14mu} {aqueous}\mspace{14mu} {phase}}$

The partition coefficient is a measure of the relative concentrations of the solute to be extracted in the two phases. The value of the acid partition coefficient is directly related to the amount of solvent that is required to effect a given extraction. Low values of the partition coefficient indicate high levels of solvent are required, and high values of the partition coefficient indicate low levels of solvent are required. Since the acid partition coefficient changes with acid concentration, the minimum amount of solvent required to effect a given amount of acid extraction also changes. Thus, the controlling solvent flow requirement for the extraction is dictated by the lowest value of the acid partition coefficient as the acid concentration varies from the high of the inlet wet acid feed to the low of the outlet acid concentration of the exiting raffinate stream.

The controlling acid partition coefficient may be defined as:

P _(cont)=minimum(P _(raff) ,P _(extr))

where

P_(raff)=acid partition coefficient at an acid concentration approaching that desired in the raffinate stream (i.e., at low acid concentration); and

P_(extr)=acid partition coefficient at an acid concentration approaching that desired in the extract stream (i.e., at high acid concentration).

The most important water-acid selectivity value is that at the extract end of the extraction cascade. It is defined as:

R _(extr) =W _(extr) /A _(extr)

where

W_(extr)=weight fraction of water in the extract product stream; and

A_(extr)=weight fraction of acid in the extract product stream.

The controlling partition coefficient, P_(cont), and extract water-to-acid ratio, R_(extr), may be combined to yield an overall extraction factor, ε, which is a simple measure of the efficacy of a given solvent for recovering lower acids from wet acid feeds in an extraction-distillation process. The extraction factor, ε, is defined as:

ε=P _(cont) /R _(extr)=(P _(cont) *A _(extr))/W _(extr)

Generally, the higher the extraction factor, the lower the capital and energy costs are for a given extraction.

Extraction solvents that exhibit the inverse behavior are also known. That is, their acid partition coefficient is lowest at the extract end of the cascade (high acid concentration) and highest at the raffinate end (low acid concentration). Examples of such solvents include nitriles, phosphate esters, phosphine oxides (U.S. Pat. Nos. 3,816,524 and 4,909,939), and amines (e.g., King, “Amine-Based Systems for Carboxylic Acid Recovery: Tertiary Amines and the Proper Choice of Diluent Allow Extraction and Recovery from Water,” CHEMTECH, Vol. 5, pp. 285-291 (1992); and Tamada et al., “Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretation of Data,” Ind. Eng. Chem. Res., Vol. 29, pp. 1327-1333 (1990)).

This inverse behavior (partition coefficient highest at low acid concentration) has also been observed for a phosphonium- and an ammonium-phosphinate ionic liquid (Blauser et al., “Extraction of butyric acid with a solvent containing ammonium ionic liquid,” Sep. Purif. Technol., Vol. 119, pp. 102-111 (2013); Martak et al., “Phosphonium ionic liquids as new, reactive extractants of lactic acid,” Chem. Papers, Vol. 60, pp. 395-98 (2006)) and a phosphonium carboxylate salt (Oliveira et al., “Extraction of L-Lactic, L-Malic, and Succinic Acids Using Phosphonium-Based Ionic Liquids,” Sep. Purif. Tech., Vol. 85, pp. 137-146 (2012)). Oliveira et al., in particular, report that P_(666,14)-decanoate fails to form a simple two-phase system in the majority of extraction trials with lactic, succinic, and malic acids. A third phase complicates the distribution and recovery of carboxylic acid.

The use of hydrophobic ionic liquids as extraction solvents has been reviewed by Poole et al., “Extraction of Organic Compounds with Room Temperature Ionic Liquids,” J. Chromatogr. (A), Vol. 1217, pp. 2268-2286 (2010). The development and advantages of phosphonium ionic liquids have been reviewed by Robertson et al., “Industrial Preparation of Phosphonium Ionic Liquids”, Green Chem., Vol. 5, pp. 143-152 (2003)), and their application to the extraction of ethanol from fermentation broths is addressed by Neves et al., “Separation of Ethanol-Water Mixtures by Liquid-Liquid Extraction Using Phosphonium-Based Ionic Liquids,” Green Chem., Vol. 13, pp. 1517-1526 (2011).

Extraction of lower carboxylic acids using imidazolium and phosphonium ionic liquids has also been reported. For acetic acid, McFarlane et al. report on bmim-NTf₂, omim-NTf₂, bmim-PF₆, P_(666,14)-LABS/nonanol, P_(444,14)-LABS/nonanol, and P_(666,14)-OSO₂Me (“Room Temperature Ionic Liquids for Separating Organics from Produced Waters,” Sep. Sci. & Tech., Vol 40., 1245-1265 (2005)). Hashikawa claims P_(222,8)-NTf₂ for acetic, propionic, and butyric acids (“Method for Producing Acetic Acid,” JP 2014/40389, Daicel, (Mar. 6, 2014)). And Matsumoto et al. report on bmim-PF₆, hmim-PF₆, and omim-PF₆ (“Extraction of Organic Acids Using Imidazolium-Based Ionic Liquids and Their Toxicity to Lactobacillus rhamnosus,” Sep. and Purif. Tech., Vol. 40, pp. 97-101 (2004)).

None of these documents, however, employed a quaternary phosphonium carboxylate with mono-functional carboxylic acids. Moreover, lower carboxylic acid partitioning was poor in the reports by McFarlane, Hashikawa, and Matsumoto. Furthermore, adding an alcohol to phosphonium ionic liquid compositions for extracting lower carboxylic acids is recognized by those skilled in the art as not being preferred, due to the formation of carboxylic ester derivatives of the alcohols with the acid extracts, especially in downstream distillation or evaporative processes for purifying the lower carboxylic acids. This exclusion is addressed by Judson King, “Acetic Acid Extraction,” Handbook of Solvent Extraction, Krieger Publ. Co. (1991).

Hashikawa, in particular, claims only using ionic liquids with fluorine-containing anions, such as bis(fluorosulfonyl)imide, bis(fluoroalkylsulfonyl)imides, tris(perfluoroalkyl)trifluorophosphates, hexafluorophosphates, tetrafluoroborates, and perfluoroalkylsulfonates. These anions add significant cost and toxicity concerns to large-scale applications. In addition, Hashikawa claims ionic liquids with phosphonium salts containing a total of only ten carbon atoms or higher. According to the data presented in the Hashikawa application, triethyl(octyl)phosphonium bis(trifluoromethylsulfonyl)-imide exhibits relatively poor extraction behavior for acetic acid, with a small two-phase region, low capacity for acetic acid, and very low partition coefficients (between about 0.06 and 0.1).

Despite the poor performance of the above reported and claimed ionic liquid systems, the extremely low vapor pressure of ionic liquids remains an attractive physical property for a lower-carboxylic-acid-extracting phase. Thus, there is a need in the art for an extraction solvent with excellent partitioning of lower carboxylic acids from aqueous solutions and that enables the simple separation of these acids via distillation from the solvent. There is also a need for extraction solvents with high extraction factors whereby C₁ to C₄ carboxylic acids can be recovered from wet acid feeds in an energy-efficient and cost-effective manner.

The present invention addresses these needs as well as others, which will become apparent from the following description and the appended claims.

SUMMARY OF THE INVENTION

The invention is as set forth in the following detailed description and the appended claims.

Briefly, in one aspect, the present invention provides a solvent for extracting a C₁ to C₄ carboxylic acid from water. The extraction solvent comprises (a) a quaternary phosphonium carboxylate salt and (b) a non-ionic organic solvent with the proviso that the non-ionic organic solvent is not the extract. The carboxylate salt has the general formula 1:

wherein

R¹, R², R³ and R⁴ are each independently a C₁ to C₂₆ hydrocarbyl group, provided that R¹, R², R³, and R⁴ collectively have a total of at least 24 carbon atoms; and

R⁵ is hydrogen or a C₁ to C₂₄ non-aromatic hydrocarbyl group.

In another aspect, the present invention provides a composition for separating a C₁ to C₄ carboxylic acid from water. The composition comprises:

(a) a quaternary phosphonium carboxylate salt according to the invention;

(b) a non-ionic organic solvent according to the invention;

(c) a C₁ to C₄ carboxylic acid; and

(d) water.

In yet another aspect, the present invention provides a process for separating a C₁ to C₄ carboxylic acid from water. The process comprises contacting a feed mixture comprising a C₁ to C₄ carboxylic acid and water with an extraction solvent according to the invention at conditions effective to form (a) an extract mixture comprising the carboxylate salt, the non-ionic organic solvent, and at least a portion of the C₁ to C₄ carboxylic acid from the feed mixture and (b) a raffinate mixture comprising water and less of the C₁ to C₄ carboxylic acid compared to the feed mixture.

In one embodiment, the present invention is directed to a process for separating acetic acid from water. The process comprises contacting a feed mixture comprising acetic acid and water with an extraction solvent comprising a quaternary phosphonium carboxylate salt according to the invention at conditions effective to form (a) an extract mixture comprising the carboxylate salt and at least a portion of the acetic acid from the feed mixture and (b) a raffinate mixture comprising water and less of the acetic acid compared to the feed mixture.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is an 1H NMR spectrum of P_(666,14)-2EH in CDCl₃ at 300 MHz from Example 2.

DETAILED DESCRIPTION OF THE INVENTION

It has been found, surprisingly, that when certain quaternary phosphonium carboxylates are combined with aqueous solutions of a lower carboxylic acid, the resulting partitioning of the lower acid into the phosphonium carboxylate phase can be fairly high, particularly when the concentration of the lower acid is low (e.g., <5 wt %). The carboxylates show superior selectivity for lower acid extraction over co-extraction of water. As a result, the extraction factor, ε, is significantly higher for the carboxylates than other classes of lower acid extraction solvents, and are thus particularly useful for recovering lower acids from wet acid streams.

Accordingly, in one aspect, the present invention provides quaternary phosphonium carboxylates that are useful for separating lower acids from aqueous streams. The carboxylates are depicted by the general formula 1:

wherein

R¹, R², R³ and R⁴ are each independently a C₁ to C₂₆ hydrocarbyl group, provided that R¹, R², R³, and R⁴ collectively have a total of at least 24 carbon atoms; and R⁵ is hydrogen or a C₁ to C₂₄ non-aromatic hydrocarbyl group.

As used herein, the term “hydrocarbyl” refers to a group containing hydrogen and carbon atoms, and may be straight-chained or branched, cyclic or acylic, and saturated or unsaturated.

Each of R¹, R², R³, and R⁴ may have the same number of carbon atoms or may be of different lengths. In one embodiment, R¹, R², R³, and R⁴ collectively have not more than 54 carbon atoms. In another embodiment, each of R¹, R², R³, and R⁴ has at least 6 carbon atoms. In other embodiments, each of R¹, R², R³, and R⁴ contains from 6 to 24 carbon atoms, 6 to 20 carbon atoms, 6 to 18 carbon atoms, 6 to 14 carbon atoms, or 8 to 14 carbon atoms.

Preferably, R⁵ has such a length that when combined with the phosphonium group [PR¹R²R³R⁴], defined as above, the carboxylate anion renders the salt hydrophobic. In one embodiment, R⁵ is a C₃ to C₉ non-aromatic hydrocarbyl group. In another embodiment, R⁵ is a C₃ to C₂₀ non-aromatic hydrocarbyl group. In other embodiments, R⁵ may contain from 1 to 20 carbon atoms, 1 to 18 carbon atoms, 1 to 16 carbon atoms, 1 to 14 carbon atoms, 1 to 12 carbon atoms, 3 to 20 carbon atoms, 3 to 18 carbon atoms, 3 to 16 carbon atoms, 3 to 14 carbon atoms, or 3 to 12 carbon atoms. R⁵ may contain functional groups such as alkoxy, olefinic, and halogen functionalities.

By “hydrophobic,” it is meant that the salt is immiscible in water at typical extraction conditions, e.g., has less than 5 wt % miscibility in water at 20° C.

The carboxylate salt is the liquid state under typical extraction conditions.

Examples of phosphonium carboxylates having the structure of formula 1 are listed in Table 1 below. Their CAS registry numbers are also listed if the compounds are known.

TABLE 1 CAS Registry Numbers of Some Known Phosphonium Carboxylates Cation Anion P₆₆₆₆ P_(666, 14) P₈₈₈₈ formate 1393375-35-3^(a) 1319099-97-2^(a) 1393375-36-4^(a) acetate 49720-18-5^(a) 872700-58-8^(a) 172682-89-2^(c) propionate NF 1393375-55-7^(a) NF acrylate NF NF NF butyrate NF 1393375-56-8^(a) NF isobutyrate NF 1393375-59-1^(a) NF methacrylate NF NF NF hexanoate NF 151196-58-0^(a) NF 2-ethylbutyrate NF NF NF octanoate NF 1393375-58-0^(a) NF 2-ethylhexanoate NF NF NF nonanoate NF NF NF decanoate NF 465527-65-5^(b) NF ^(a) Pure & Appl. Chem., Vol. 84, p. 723 (2012). ^(b) Green Chem., Vol. 7, p. 855 (2005). ^(c)U.S. Pat. No. 5,663,422 (Eastman Chemical; 1997). NF = not found.

The carboxylate derivatives in Table 1 that do not have CAS registration numbers are previously unknown and, thus, are specifically contemplated as being within the scope of the present invention.

Mixtures containing more than one quaternary cation and more than one carboxylate anion are also useful for the extraction of lower acids from aqueous solutions and, therefore, are also contemplated as being within the scope of the present invention.

In one embodiment, the carboxylate salt comprises a tetraalkylphosphonium salt of formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acids, n-octanoic, 2-ethylhexanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, palmitic acid, stearic acid, oleic acid, linolenic acid, mixed vegetable-derived acids, or chloroacetic acid.

In another embodiment, the carboxylate salt comprises a trihexyl(tetradecyl)phosphonium salt of formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic, 2-ethylhexanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, palmitic acid, stearic acid, oleic acid, linolenic acid, mixed vegetable-derived acids, or chloroacetic acid.

In a further embodiment, the carboxylate salt comprises trihexyl(tetradecyl)phosphonium carboxylate. The carboxylate anion may selected from the group consisting of formate, acetate, propionate, acrylate, butyrate, isobutyrate, methacrylate, hexanoate, 2-ethylbutyrate, octanoate, 2-ethylhexanoate, nonanoate, and decanoate.

Some phosphonium carboxylates having the structure of formula 1 are commercially available, while others may be produced by known methods from readily available precursors. See, e.g., Kogelnig et al., “Greener Synthesis of New Ammonium Ionic Liquids and their Potential as Extracting Agents,” Tetrahedron Letters, Vol. 49, pp. 2782-2785 (2008) and Ferguson et al., “A Greener, Halide-Free Approach to Ionic Liquid Synthesis,” Pure & Appl. Chem., Vol. 84, pp. 723-744 (2012)). The former approach involves the metathesis of an alkali carboxylate with a quaternary ammonium halide, while the latter approach employs an ion-exchange resin and in our experience (see Examples 1 and 2 below) produces ionic liquids with lower amounts of residual halide. An example of a readily available precursor is trihexyl(tetradecyl)phosphonium chloride.

So while synthetic methods of a general nature are known for making quaternary phosphonium carboxylates, a relatively small number have been prepared and their application to the extraction of low acids from aqueous streams is unknown, with one exception as reported by Oliveira et al. and described herein: P₆₆₆₁₄-decanoate for the extraction of lactic, malic, and succinic acids. In one embodiment, therefore, the invention excludes the use of P_(666,14)-decanoate alone for extracting aqueous solutions of lactic, malic, or succinic acids.

In a second aspect, the invention provides a composition A:

Composition A represents a unique biphasic mixture comprising a quaternary phosphonium carboxylate according to the general formula 1, a lower acid, and water. Other species may also be present. Composition A is useful for separating the lower acid from water.

As noted above, lower acids refer to C₁ to C₄ carboxylic acids. By way of example, the carboxylic acids may be monocarboxylic acids, alkoxycarboxylic acids, or halogen-containing carboxylic acids. Examples of such acids include formic acid, acetic acid, propionic acid, acrylic acid, n-butyric acid, isobutyric acid, methacrylic acid, chloroacetic acid, dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid, methoxyacetic acid, and the like. In one embodiment, the C₁ to C₄ carboxylic acid is a monocarboxylic acid without a hydroxyl group.

Examples of processes that produce diluted aqueous carboxylic acid-containing streams (i.e., comprising less than 1 weight percent to 60 weight percent of C₁ to C₄ carboxylic acids in an aqueous mixture, which may be referred to as “wet acid feeds”) include the production of cellulose esters or terephthalic acid, the production of ketene or higher ketenes from high temperature dehydration of carboxylic acids and anhydrides, the hydrolysis of poly(vinylacetate), the production of Fischer-Tropsch liquids, oil and gas production (yielding “produced waters”), the ketonization of carboxylic acids to ketones, the oxidation of ethylene to acetaldehyde by the Wacker process, the oxidation of propylene to acrylic acid, the oxidation of oxo aldehydes to their carboxylic acids, hydrocarboxylation of formaldehyde with water and carbon monoxide, the oxidation of isobutylene to methacrylic acid, pyroligneous acids, fermentations broths, vinegar streams, and the like. Vinegar streams refer to aqueous streams containing acetic acid. In one embodiment, the wet acid feed is derived from the production of cellulose esters.

The wet acid feed may comprise from 0.5 to 60 weight percent of one or more of the C₁ to C₄ carboxylic acids. More preferably, the wet acid feed comprises 0.5 to 35 weight percent of the C₁ to C₄ carboxylic acids. Because of the unusually high acid partition coefficients of the carboxylate salts of the invention even at low acid concentrations, the extraction solvent of the instant invention may be used advantageously to extract lower acids at concentrations as low as 0.5 weight percent in the wet acid feed.

As used herein, the terms “feed” and “feed mixture” are intended to have their commonly understood meaning in the liquid-liquid extraction art, which is the solution that contains the materials to be extracted or separated. In the present invention, one example of a “feed” is a mixture composed of one or more of formic, acetic, propionic, acrylic, n-butyric, isobutyric, methacrylic, methoxyacetic, chloroacetic, dichloroacetic, trichloroacetic, and trifluoroacetic acids in water. In the present invention, the terms “feed” and “feed mixture” are synonymous with “aqueous acid stream,” “weak acid stream,” and “wet acid feed.”

The term “extraction solvent,” as used herein, is intended to be synonymous with the term “extractant” and is intended to mean the water-immiscible or hydrophobic liquid that is used in the extraction process to extract materials or solutes from the feed.

The weight ratio of carboxylate to wet acid feed in composition A may vary over a wide range. For example, the ratio may range from 0.2 to 10:1 or, more preferably, from 0.3 to 4:1.

A feature of composition A is that it separates into two phases, an aqueous and an organic phase, with the lower acid distributed between them. The biphasic nature of composition A is desirable in order to physically separate the lower acid from the aqueous solution. The amount of lower acid distributed between the phases is only limited by the biphasic property of the system. Preferably, the lower acid amount does not exceed a level at which the biphasic nature of the composition is lost. Likewise, other materials may also be present, but only to the extent that the biphasic nature of the system is retained. Complex systems that form more than two phases are not preferred, since such a system can obscure the effective separation of the lower acid.

Quaternary phosphonium carboxylates that form greater than two-phase systems, emulsions, or other complex mixtures may be simplified by adding a hydrophobic, non-ionic organic co-solvent to the quaternary phosphonium carboxylate extracting phase.

Thus, in a third aspect, the present invention provides a solvent for extracting lower carboxylic acids (C₁-C₄) from water. The extraction solvent comprises the phosphonium carboxylate defined by the general formula 1 above and a non-ionic organic (NIO) solvent. The NIO solvent is not the extract (i.e., the C₁-C₄ carboxylic acid to be separated). Rather, it is separate from and in addition to the lower carboxylic acid to be separated.

The extraction solvent may comprise two or more of the carboxylate salts.

The NIO solvent is preferably selected to impart desirable physical properties to the extraction solvent, such as lower viscosity or higher hydrophobicity or to provide low-boiling azeotropes with water as described above and illustrated in, for example, U.S. Pat. Nos. 1,861,841; 1,917,391; 2,028,800; 3,052,610; 5,662,780; 2,076,184; and 2,204,616, to enable drying of the lower carboxylic acid in a subsequent purification step.

Examples of such hydrophobic NIO solvents include ketones, aromatic hydrocarbons, saturated hydrocarbons, ethers, esters, chlorinated hydrocarbons, nitriles, and higher carboxylic acids.

Fatty alcohols, such as nonanol, are not preferred as these may complicate the separation of the lower acids by forming esters during extraction or subsequent purification. In one embodiment, the NIO solvent excludes fatty alcohols, such as nonanol.

Likewise, care should be exercised when selecting specific compounds from any of the above classes of co-solvent, which, in combination with the lower acids or water, may form azeotropes or may be difficult to separate from the lower acids.

Preferred non-ionic organic solvents form minimum-boiling azeotropes with water, but do not form azeotropes with the lower acid.

In one embodiment, the NIO solvent has 4 to 20 carbon atoms. In another embodiment, the NIO solvent has 4 to 18 carbon atoms. In other embodiments, the NIO solvent has 4 to 16 carbon atoms, 4 to 14 carbon atoms, 4 to 12 carbon atoms, 5 to 20 carbon atoms, 5 to 18 carbon atoms, 5 to 16 carbon atoms, 5 to 14 carbon atoms, or 5 to 12 carbon atoms.

In one particular embodiment, the NIO solvent comprises a higher carboxylic acid. Remarkably, it has been found that as little as 10 mole percent of a higher carboxylic acid in the extraction solvent can significantly enhance the partition coefficient of the lower acids, especially at low concentration in wet acid streams.

As used herein, “higher carboxylic acids” refer to a carboxylic acid having 4 to 20 carbon atoms. In the case where the carboxylic acid to be separated has 4 carbon atoms, the “higher carboxylic acid” contains at least 5 carbon atoms or has a sufficiently different boiling point (e.g., +/−2° C.) from the lower acid to be separated such that the two may be separated from one another by simple distillation.

In one embodiment, the higher carboxylic acid is selected from the group consisting of n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic, 2-ethylhexanoic acids, nonanoic acids, decanoic acids, dodecanoic acids, stearic acid, oleic acid, linolenic acid, and mixed vegetable-derived acids.

In another embodiment, the higher carboxylic acid is selected from the group consisting of n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic acid, and 2-ethylhexanoic acid.

Preferred NIO esters are those containing four to six carbon atoms such as ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, and i-propyl propionate.

Preferred NIO ketones are those containing five to nine carbon atoms such as 2-pentanone, 3-pentanone, 3-methyl-2-butanone, 2-hexanone, 2-heptanone, cyclohexanone, 4-methyl-2-pentanone, 2,4-dimethyl-3-pentanone, 5-methyl-2-hexanone, 4-heptanone, 2-octanone, 5-nonanone, 2,8-dimethyl-4-heptanone, 3,3,5-trimethyl cyclohexanone, and isophorone.

Preferred NIO ethers are those containing four to eight carbon atoms such as diethyl ether, methyl propyl ether, dipropyl ether, di-isopropyl ether, methyl tert butyl ether, tertiary amyl methyl ether, and ethyl butyl ether.

Preferred NIO aromatic hydrocarbons include toluene, m-xylene, p-xylene, and o-xylene.

Preferred NIO chlorinated hydrocarbons include methylene chloride, chloroform, dichloroethane, ethylene chloride, carbon tetrachloride, and chlorinated derivatives of benzene.

Preferred NIO nitriles include valeronitrile and nitriles that are higher boiling than valeronitrile, such as hexanenitrile and benzonitrile.

In one embodiment, the hydrophobic NIO solvent is selected from the group consisting of methyl isobutyl ketone, toluene, isopropyl acetate, and methyl t-butyl ether.

In another embodiment, the hydrophobic NIO solvent is a fatty carboxylic acid, such as butyric, pentanoic, hexanoic, heptanoic, octanoic, nonanoic acids, and isomeric forms of C₄-C₉ carboxylic acids.

The extraction solvent according to the invention and compositions containing the same may include two or more of the NIO solvents. Desirable physical properties of the claimed systems may best be achieved by employing mixtures of the hydrophobic solvents.

The extraction solvent of the invention may comprise from 0 to 90, 10 to 90, 20 to 90, 30 to 90, 40 to 90, or 50 to 90 weight percent of the NIO solvent. The extraction solvent may also comprise from 0 to 60, 10 to 60, 20 to 60, 30 to 60, 40 to 60, or 50 to 60 weight percent of the NIO solvent. The balance of the extraction solvent may be composed of the carboxylate salt according to the invention.

The NIO solvent may be combined with the carboxylate salt before introduction into the extraction vessel. Alternatively, the NIO solvent may be introduced separately into the extraction vessel. In one embodiment, the NIO solvent may be introduced as a second solvent feed on the other side of the extraction cascade from the wet acid feed, such as in a fractional extraction mode, wherein the NIO solvent helps to wash any carboxylate from the final raffinate product stream.

The carboxylate or mixture of carboxylates of the present invention may be mixed with one or more of the NIO solvents to form the extraction solvent in any known manner.

In a fourth aspect, the invention provides a composition B:

Composition B comprises the carboxylate salt according to formula 1, the NIO solvent, a C₁ to C₄ carboxylic acid, and water. It is useful for separating the lower carboxylic acid from water and optionally purifying the lower acid.

Composition B may contain more than one of the carboxylate salt, more than one of the NIO solvent, and/or more than one of the lower acid. The carboxylate salt, NIO solvent, and lower acid may be any of those described herein.

In one embodiment, composition B has only two liquid phases.

A feature of composition B is that the C₁ to C₄ carboxylic acid may be recovered by distillation at atmospheric pressure or lower.

The weight ratio of the extraction solvent (carboxylate and NIO solvent) to wet acid feed in composition B may vary over a wide range. For example, the ratio may range from 0.2 to 10:1 or, more preferably, from 0.3 to 4:1.

In a fifth aspect, the present invention provides a process for separating a C₁ to C₄ carboxylic acid from water. The process includes the step of contacting a feed mixture comprising a C₁ to C₄ carboxylic acid and water with an extraction solvent comprising a quaternary phosphonium carboxylate salt according to the invention and a non-ionic organic solvent according to the invention at conditions effective to form (a) an extract mixture comprising the carboxylate salt, the non-ionic organic solvent, and at least a portion of the C₁ to C₄ carboxylic acid from the feed mixture and (b) a raffinate mixture comprising water and less of the C₁ to C₄ carboxylic acid compared to the feed mixture.

The extraction of the feed mixture (i.e., the contacting step) can be carried out by any means known in the art to intimately contact two immiscible liquid phases and to separate the resulting phases after the extraction procedure. For example, the extraction can be carried out using columns, centrifuges, mixer-settlers, and miscellaneous devices. Some representative examples of extractors include unagitated columns (e.g., spray, baffle tray and packed, perforated plate), agitated columns (e.g., pulsed, rotary agitated, and reciprocating plate), mixer-settlers (e.g., pump-settler, static mixer-settler, and agitated mixer-settler), centrifugal extractors (e.g., those produced by Robatel, Luwesta, deLaval, Dorr Oliver, Bird, CINC, and Podbielniak), and other miscellaneous extractors (e.g., emulsion phase contactor, electrically enhanced extractors, and membrane extractors). A description of these devices can be found in the “Handbook of Solvent Extraction,” Krieger Publishing Company, Malabar, F L, pp. 275-501 (1991). The various types of extractors may be used alone or in any combination.

The extraction may be conducted in one or more stages. The number of extraction stages can be selected based on a number of factors, such as capital costs, achieving high extraction efficiency, ease of operability, the stability of the feed and the extraction solvent, and the extraction conditions. The extraction also can be conducted in a batch or continuous mode of operation. In a continuous mode, the extraction may be carried out in a co-current, a counter-current manner, or as a fractional extraction in which multiple solvents and/or solvent feed points are used to help facilitate the separation. The extraction process also can be conducted in a plurality of separation zones that can be in series or in parallel.

The extraction may be carried out at an extraction solvent:feed mixture weight ratio of, for example, 0.2 to 10:1 or, more preferably, 0.3 to 4:1.

The extraction typically can be carried out at a temperature of 10 to 140° C. For example, the extraction can be conducted at a temperature of 30 to 110° C. The desired temperature range may be constrained further by the boiling point of the extractant components or water. Generally, it is undesirable to operate the extraction under conditions where the extractant boils. In one embodiment, the extractor can be operated to establish a temperature gradient across the extractor in order to improve the mass transfer kinetics or decantation rates.

If the temperature chosen for the extraction is greater than the normal boiling point of any of the lower acid to be extracted, any of the components comprising the extraction solvent, or water; then the extractor may be run under sufficient pressure to suppress boiling of any of aforementioned components. The extraction typically can be carried out at a pressure of 1 bara to 10 bara, or 1 bara to 5 bara.

The separation process according to the invention may further include the steps of separating the extract from the raffinate and recovering the C₁ to C₄ carboxylic acid from the extract by distillation at atmospheric pressure or lower. Any known method from separating a liquid extract from a raffinate may be used. Likewise, any known distillation technique may be used to recover the lower acid from the extraction solvent.

In one embodiment, the present invention provides a process for separating acetic acid from water. The process comprises contacting a feed mixture comprising acetic acid and water with an extraction solvent comprising a quaternary phosphonium carboxylate salt according to the invention at conditions effective to form (a) an extract mixture comprising the carboxylate salt and at least a portion of the acetic acid from the feed mixture and (b) a raffinate mixture comprising water and less of the acetic acid compared to the feed mixture.

This acetic acid separation process may be carried out using any of the modes described herein above.

The extraction solvent used in this process may further comprise one or more of the NIO solvents according to the invention.

The acetic acid separation process may further include the steps of separating the extract mixture from the raffinate mixture and recovering the acetic acid from the extract mixture by distillation at atmospheric pressure or lower.

These additional steps may also be carried out as described herein above.

The present invention includes and expressly contemplates any and all combinations of embodiments, features, and/or ranges disclosed herein. That is, the invention may be defined by any combination of embodiments, features, and/or ranges mentioned herein.

As used herein, the indefinite articles “a” and “an” mean one or more, unless the context clearly suggests otherwise. Similarly, the singular form of nouns includes their plural form, and vice versa, unless the context clearly suggests otherwise.

As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself, or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination.

While attempts have been made to be precise, the numerical values and ranges described herein should be considered to be approximations (even when not qualified by the term “about”). These values and ranges may vary from their stated numbers depending upon the desired properties sought to be obtained by the present invention as well as the variations resulting from the standard deviation found in the measuring techniques. Moreover, the ranges described herein are intended and specifically contemplated to include all sub-ranges and values within the stated ranges. For example, a range of 50 to 100 is intended to describe and include all values within the range including sub-ranges such as 60 to 90 and 70 to 80.

The content of all documents cited herein, including patents as well as non-patent literature, is hereby incorporated by reference in their entirety. To the extent that any incorporated subject matter contradicts with any disclosure herein, the disclosure herein shall take precedence over the incorporated content.

This invention can be further illustrated by the following examples of preferred embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention.

EXAMPLES

Abbreviations used in the following examples are summarized in Table 2.

TABLE 2 Abbreviations Compound Abbreviation acetic acid HOAc propionic acid HOPr n-butyric acid n-HOBu isobutyric acid i-HOBu 2,2-dimethylbutyric acid 2,2-DMB acid 2-ethylbutyric acid 2EB acid 2-ethylhexanoic acid 2EH acid butyronitrile PrCN methyl t-butyl ether MTBE t-amyl methyl ether TAME 4-heptanone DPK ethyl acetate EtOAc i-propyl acetate i-PrOAc n-propyl acetate n-PrOAc n-butyl acetate n-BuOAc i-butyl acetate i-BuOAc 2-pentanone MPK 4-methyl-2-pentanone MIBK 5-methyl-2-hexanone MIAK 2-heptanone MAK tributyl phosphate TBP triethylhexyl phosphate TEHP Cyanex 923: a mixture of trialkyl phosphine oxides with octyl and hexyl groups C923 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide emim-NTf₂ 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide bmim-NTf₂ 1-butyl-3-methyl imidazolium acetate bmim-OAc 1-butyl-3-methyl imidazolium bis(trifluoroethylsulfonyl)imide bmim-BETI 1-butyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate bmim-FAP 1-hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide hmim-NTf₂ 1-octyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide omim-NTf₂ 1-octyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide omim-BETI 1-decyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide C₁₀mim-NTf₂ 1-butyl-2,3-dimethyl imidazolium bis(trifluoromethylsulfonyl)imide C₄mmim-NTf₂ 1-(2-methoxyethyl)-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate MeOEtmim-FAP 1-(8-hydroxyoctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide HOC₈mim-NTf₂ tridecafluoro-octyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide C₈—H₄F₁₃mim-NTf₂ dimethylaminoethyl-dimethylethylammonium bis(trifluoromethyl)sulfonylimide iPr₂N(CH₂)₂mim-NTf₂ 1-butyl pyridinium bis(trifluoromethylsulfonyl)imide bpyr-NTf₂ 1-(2-methoxyethyl)-pyridinium tris(pentafluoroethyl)trifluorophosphate MeOEtpyr-FAP 1-(4-cyanobutyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (4-CN)bmim-NTf₂ trimethyl(butyl)ammonium bis(trifluoromethylsulfonyl)imide N₁₁₁₄-NTf₂ trimethyl(octyl)ammonium bis(trifluoromethylsulfonyl)imide N₁₁₁₈-NTf₂ 1-(2-diisopropylaminoethyl) dimethylethylammonium iPr₂N(CH₂)₂N₂₁₁-NTf₂ bis(trifluoromethyl)sulfonylimide dimethylaminoethyl-dimethylethylammonium bis(trifluoromethyl)sulfonylimide Me₂N(CH₂)₂N₂₁₁-NTf₂ choline bis(trifluoromethylsulfonyl)imide choline-NTf₂ 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide C4mpyrr-NTf₂ N-trimethylbetainium bis(trifluoromethylsulfonyl)imide C₁Hbet-NTf₂ triethyl(octyl)phosphonium bis(trifluoromethylsulfonyl)imide P₂₂₂₈-NTf₂ trioctyl(methyl)phosphonium bis(trifluoromethylsulfonyl)imide P₈₈₈₁-NTf₂ 1-(2-diisopropylaminoethyl) trioctylphosphonium bis(trifluoromethyl)sulfonylimide iPr₂N(CH₂)₂P₈₈₈-NTf₂ trihexyl(tetradecyl)phosphonium chloride P_(666, 14)-Cl trihexyl(tetradecyl)phosphonium hydroxide P_(666, 14)-OH trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide P_(666, 14)-NTf2 trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate P_(666, 14)-FAP trihexyl(tetradecyl)phosphonium formate P_(666, 14)-formate trihexyl(tetradecyl)phosphonium acetate P_(666, 14)-OAc trihexyl(tetradecyl)phosphonium isobutyrate P_(666, 14)-isobutyrate trihexyl(tetradecyl)phosphonium hexanoate P_(666, 14)-hexanoate trihexyl(tetradecyl)phosphonium 2-ethylbutyrate P_(666, 14)-2EB trihexyl(tetradecyl)phosphonium 2,2-dimethylbutyrate P_(666, 14)-2,2-DMB trihexyl(tetradecyl)phosphonium octanoate P_(666, 14)-octanoate trihexyl(tetradecyl)phosphonium 2-ethylhexanoate P_(666, 14)-2EH trihexyl(tetradecyl)phosphonium dodecanoate P_(666, 14)-dodecanoate trihexyl(tetradecyl)phosphonium succinate P_(666, 14)-succinate

Example 1 Synthesis of P_(666,14)-OH

P_(666,14)-Cl (81.70 g, 157.32 mmol) was dissolved in methanol (19.78 g, 617.35 mmol), and the solution was settled for further ion-exchange reaction. A column was packed with ion exchange resin (170 cm³, Amberlite IRN-78), and the resin was washed with methanol. The P_(666,14)-Cl solution was then added slowly to the column, with the flow rate controlled at 1 droplet/2 s. Thereafter, methanol (80 mL, 1977.03 mmol) was added slowly into the column to wash out all the P_(666,14)-OH into a 500 mL single-neck round bottom flask. After all the solution containing P_(666,14)-OH in the column was collected, 5 droplets from the solution were mixed with 3 droplets of HNO₃ and 3 droplets of AgNO₃ to test if all of the halide had been exchanged. Where necessary, this procedure was repeated.

The concentration of P_(666,14)-OH in solution was analyzed by ¹H NMR, and a wt % of P_(666,14)-OH in methanol was determined by the relative integrations of the acidic protons versus the methyl group of methanol versus the hydrocarbon chains of the phosphonium salt.

Example 2 Synthesis of P_(666,14)-2EH

From the calculated weight fraction of P_(666,14)-OH in methanol determined in Example 1 (weight fraction of P_(666,14)-OH=0.3032), the acid needed to neutralize the P_(666,14)-OH was calculated as follows:

-   -   MW of 2-EH Acid=144.21;     -   MW of P_(666,14)-OH=500.86;

weight of 2-EH Acid needed={[(weight of hydroxide MeOH solution)*(wt % of P_(666,14)-OH)]/(MW of P_(666,14)-OH)}*(MW of 2-EH Acid)={[(252.36 g)*[0.3032/(500.86 g/mol)]}*(144.12 g/mol)=22.017 g.

2-Ethylhexanoic acid (22.04 g, 152.93 mmol) was added into the P_(666,14)-OH solution, and the solution was then stirred for 6 h. The solution was then dried in a rotary evaporator at 40° C. for 6 h. Further drying was conducted in a high vacuum drying system for 6 h. The formed carboxylate was then ready for liquid-liquid extraction (LLE) experimentation and further characterization, such as water content measurement, differential scanning calorimetry, thermogravimetric analysis, and elemental analysis.

The ¹H NMR Spectrum of pure P_(666,14)-2EH in CDCl₃ at 300 MHz is shown in the FIGURE.

Example 3 Extraction of Acetic Acid Using Representative NIO Solvents

Tie line data at both high (typically around 16-20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 1 to 5 wt % of HOAc in the organic phase) were measured for each solvent at the temperature given in Table 3.

Roughly equal masses of water and solvent were added to a glass vial. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 3.

TABLE 3 Acetic Acid Extraction Factors of Non-Ionic Organic Solvents T HOAc at P_(cont) Extraction Solvent (° C.) P_(cont) (wt %) R_(extr) Factor (ε) EtOAc 25 0.99 1.5 0.98 1.01 n-BuOAc 40 0.41 1.6 0.52 0.79 nPrOAc 40 0.51 2.2 0.70 0.73 iPrOAc 40 0.54 1.8 0.65 0.83 PrCN 20 2.85 17.0 0.7 4.07 2EH Acid 40 0.32 1.3 0.29 1.10 MTBE 40 0.7 2.0 0.5 1.40 TAME 40 0.42 1.5 0.34 1.24 2-hexanone^(a) 35 0.91 3.9 0.97 0.93 MIBK 35 0.65 1.6 0.53 1.23 MAK 40 0.49 1.7 0.41 1.20 MIAK 40 0.46 1.7 0.34 1.35 DPK 40 0.32 1.3 0.31 1.03 isophorone 40 1.1 1.2 0.67 1.64 ^(a)Data taken from J. Chem. Eng. Data, 46, 2001, 1450-56.

Although PrCN has a relatively high extraction factor, it has the same boiling point as acetic acid, forms an azeotrope with acetic acid, and is thus very difficult to separate from acetic acid.

Example 4 Extraction of Acetic Acid Using Phosphate Ester Solvents

Tie line data at both high (typically around 15-20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 1 to 2 wt % of HOAc in the organic phase) were measured for each solvent (either a pure phosphate ester or a mixture of phosphate ester with a NIO solvent) at the temperature given in Table 4.

Roughly equal masses of water and solvent were added to a glass vial. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 4.

TABLE 4 Acetic Acid Extraction by Phosphate-Ester-Containing Compositions at 40° C. HOAc at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) tributyl phosphate 0.78 18.1 0.39 2.00 25 wt % TBP, 75 wt % MTBE 0.94 19.7 0.52 1.81 25 wt % TBP, 75 wt % iPrOAc 0.80 18.1 0.61 1.31 triethylhexyl phosphate 0.33 16.2 0.12 2.75 25 wt % TEHP, 75 wt % MTBE 0.71 16.9 0.30 2.37 25 wt % TEHP, 75 wt % iPrOAc 0.62 15.8 0.39 1.59

As expected from Wardell and King (“Solvent Equilibria for Extraction of Carboxylic Acids from Water,” J. Chem. and Eng. Data, Vol. 23, No. 2, pp. 144-148 (1978)), the extraction factors for phosphate ester solvents are somewhat improved over those of NIO solvents.

Example 5 Extraction of Acetic Acid Using Cyanex 923

Tie line data at both high (typically around 15-20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 1 to 2 wt % of HOAc in the organic phase) were measured for each solvent (either commercially available Cyanex 923 or a mixture of Cyanex 923 with a non-ionic organic solvent) at the temperature given in Table 5.

Roughly equal masses of water and solvent were added to a glass vial. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 5.

TABLE 5 Acetic Acid Extraction Factors for Cyanex-Containing Compositions at 40° C. HOAc at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) Cyanex 923 0.94 19.0 0.26 3.62 75 wt % MTBE/25 wt % C923 0.89 20.0 0.36 2.47 75 wt % iPrOAc/25 wt % C923 0.77 20.0 0.43 1.79

Example 6 Extraction of Acetic Acid Using Hydrophobic Liquid Salts

Tie line data at both high (typically around 7-25 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured for each solvent at the temperature specified in Table 6.

Some solvents showed extremely low acid partition coefficients, and the two-phase region did not extend much above about 7 wt % of acetic acid. Equilibrium data were measured for each compound in the following manner.

Three grams of the solvent were pipetted into a jacketed glass cell, wherein three grams of an aqueous mixture of acetic acid (prepared to yield either high or low acid concentration data) were added. A stir bar was introduced to the vial and the contents sealed with a plastic cap and a layer of parafilm tape. The cell was maintained at the desired temperature by means of a thermostatted fluid circulating through the cell jacket. The mixture was agitated vigorously for 1.5 hours and then allowed to separate into clear phases while maintaining the specified temperature without stirring. After a six hour settling time, each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 6.

TABLE 6 Extraction of Acetic Acid in a Selection of Hydrophobic Solvents at 20° C. HOAc at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) emim-NTf₂ 0.30 7.1 0.85 0.35 bmim-NTf₂ 0.21 1.4 0.69 0.31 bmim-NTf₂ ^(b) 0.24 5.4 0.79 0.30 bmim-FAP 0.00 1.5 0.17 0.00 bmim-FAP^(b) 0.06 1.7 0.33 0.18 bmim-BETI 0.06 0.6 0.39 0.15 hmim-NTf₂ 0.19 4.6 0.56 0.34 omim-NTf₂ 0.18 0.9 0.47 0.38 omim-NTf₂ ^(a) 0.12 11.8 0.68 0.17 omim-BETI 0.05 0.5 0.50 0.11 C₁₀mim-NTf₂ 0.13 3.6 0.39 0.34 C₄mim-NTf₂ 0.13 1.1 0.59 0.22 iPr₂N(CH₂)₂mim-NTf₂ 0.18 4.5 0.88 0.20 MeOEtmim-FAP 0.04 0.3 0.23 0.15 4CNbmim-NTf₂ 0.65 11.0 0.82 0.79 HOC₈mim-NTf₂ 0.54 7.6 0.98 0.55 (C₆F₁₃)—(C₂H₄)mim-NTf₂ 0.08 0.7 0.60 0.13 C₄mpyrr-NTf₂ 0.33 9.5 0.45 0.74 bpyr-NTf₂ 0.22 5.1 0.69 0.31 MeOEtpyr-FAP 0.06 0.2 0.23 0.26 N₁₁₁₄-NTf₂ 0.13 1.9 0.84 0.16 N₁₁₁₈-NTf₂ 0.06 1.2 0.47 0.14 Me₂N(CH₂)₂N₂₁₁-NTf₂ 0.53 9.5 2.00 0.26 iPr₂N(CH₂)₂N₂₁₁-NTf₂ 0.14 4.0 0.77 0.19 choline-NTf₂ 0.82 14.0 1.64 0.50 C₁Hbet-NTf₂ 0.76 12.7 1.48 0.51 P₂₂₂₈-NTf₂ ^(b) 0.10 2.8 0.33 0.30 P₄₄₄₄-2EH 0.52 7.3 1.452 0.36 P₈₈₈₁-NTf₂ 0.05 0.8 0.43 0.13 iPr₂N(CH₂)₂P₈₈₈-NTf₂ 0.21 5.4 0.74 0.28 P_(666,14)-Cl 0.38 8.5 0.63 0.60 P_(666,14)-NTf₂ 0.06 1.8 0.88 0.07 P_(666,14)-FAP 0.02 0.1 0.23 0.10 ^(a)Equilibration at 75° C. rather than 20° C. ^(b)Data taken from Hashikawa, JP Appl. Kokai 2014/40389.

Example 7 Extraction of Acetic Acid Using Tetraalkylphosphonium Carboxylates

Tie line data at both high (typically around 9-21 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 7.

Roughly equal masses of water and solvent were added to a glass vial. All extraction solvents comprised essentially 100 mole percent of the carboxylate. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 7.

TABLE 7 Acetic Acid Extraction by Different Phosphonium Carboxylates at 20° C. HOAc at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) P_(666,14)-formate 2.47 19.7 0.28 8.73 P_(666,14)-acetate 4.98 9.1 1.08 4.59 P_(666,14)-hexanoate 3.32 20.3 0.25 13.48 P_(666,14)-2EB 2.15 21.6 0.24 8.91 P_(666,14)-2,2-DMB 2.41 20.3 0.29 8.32 P_(666,14)-octanoate 3.17 19.9 0.24 12.99 P_(666,14)-2EH 3.24 20.4 0.21 15.21 P_(666,14)-dodeconoate 2.62 20.2 0.23 11.17 P_(666,14)-succinate 4.72 11.9 0.58 8.07

Example 8 Extraction of C₁ to C₄ Carboxylic Acids Using P_(666,14)-2EH

Tie line data at both high (typically around 16-25 weight percent of lower acid in the organic phase) and low acid concentrations (typically around 0.2 to 5 weight percent in the organic phase) were measured at 20° C. for each acid listed in Table 8.

Roughly equal masses of water and solvent were added to a glass vial. All extraction solvents comprised essentially 100 mole percent of the carboxylate. The acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the lower acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and lower acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acid weight ratio at high acid concentration, R_(extr). Results are given in Table 8.

TABLE 8 Extraction of a Variety of Carboxylic Acids by P_(666,14)-2EH Carboxylic Acid at P_(cont) Extraction Solute Acid P_(cont) (wt %) R_(extr) Factor (ε) formic 1.35 16.5 0.19 7.03 acetic 3.24 20.4 0.21 15.21 propionic 7.64 25.1 0.16 47.28 n-butyric 4.64 22.0 0.17 27.05 acrylic 4.77 20.9 0.16 28.49

Example 9 Extraction of Acetic Acid Using P_(666,14)-2EH and/or NIO Co-Solvent

Tie line data at both high (typically around 20 to 25 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 9.

Roughly equal masses of water and extraction solvent comprising different amounts of the carboxylate and the NIO solvents were added to a glass vial. Both n-hexanoic acid and 2-ethylhexanoic acid (2-EH acid) were tested as the NIO solvent. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid (wt %). These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 9.

Extraction results with 2-EH acid alone and n-hexanoic acid alone are included in Table 9 for comparison.

TABLE 9 Effect of NIO Solvent Identity and Concentration on Acetic Acid Extraction HOAc at Carboxylate P_(cont) Extraction Solvent NIO Solvent P_(cont) (wt %) R_(extr) Factor (ε) none 100 mol % 2-EH 0.32 0.32 0.29 1.10 acid^(a) none 100 mol % 0.56 1.8 1.07 0.53 n-hexanoic acid P_(666,14)-2EH none 3.24 20.4 0.21 15.21 P_(666,14)-2EH 10 mol % 2-EH acid 2.19 21.7 0.21 10.40 P_(666,14)-2EH 30 mol % 2-EH acid 1.80 19.7 0.22 8.18 P_(666,14)-2EH 50 mol % 2-EH acid 0.86 23.9 0.22 3.87 P_(666,14)-2EH 80 mol % 2-EH acid 0.53 25.8 0.30 1.79 P_(666,14)-2EH 90 mol % 2-EH acid 0.48 25.0 0.46 1.05 P_(666,14)-2EH 10 mol % 2.26 20.9 0.22 10.03 n-hexanic acid P_(666,14)-2EH 20 mol % 1.97 20.5 0.22 8.73 n-hexanoic acid P_(666,14)-2EH 30 mol % 2.00 19.6 0.24 8.31 n-hexanoic acid P_(666,14)-2EH 50 mol % 1.15 20.5 0.27 4.21 n-hexanoic acid ^(a)Equilibrated at 40° C.

Example 10 Extraction of Acetic Acid Using P_(666,14)-n-Hexanoate and/or n-Hexanoic Acid

Tie line data at both high (typically around 20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 10.

Roughly equal masses of water and extraction solvent comprising different amounts of carboxylate and NIO solvent were added to a glass vial. N-hexanoic acid was tested as the NIO solvent. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 10.

Extraction results with n-hexanoic acid alone are included in Table 10 for comparison.

TABLE 10 Effect of n-Hexanoic Acid on Acetic Acid Extraction by P_(666,14)-Hexanoate Amount of n-Hexanoic Acid HOAc at P_(cont) Extraction (mole %) P_(cont) (wt %) R_(extr) Factor (ε) none 3.32 20.3 0.25 13.48 30 2.97 19.3 0.26 11.36 50 1.80 19.6 0.27 6.63 70 1.15 15.4 0.43 2.69 90 0.65 14.7 0.86 0.76 95 0.68 14.3 1.19 0.57 100  0.56 1.8 1.07 0.53

Example 11 Extraction of Acetic Acid Using P_(666,14)-2EB and/or 2-EB Acid

Tie line data at both high (typically around 14 to 20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 11.

Roughly equal masses of water and extraction solvent comprising different amounts of the carboxylate and NIO solvent were added to a glass vial. 2-Ethylbutyric acid was tested as the NIO solvent. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 11.

TABLE 11 Effect of 2-EB acid on Acetic Acid Extraction by P_(666,14)-2EB Amount of HOAc at 2-EB Acid P_(cont) Extraction (mole %) P_(cont) (wt %) R_(extr) Factor (ε) none 2.15 21.6 0.24 8.91 10 4.75 20.7 0.24 19.80 30 2.76 22.6 0.26 10.58 50 2.05 19.9 0.29 7.05 70 1.59 14.4 0.50 3.18 80 1.01 14.4 0.63 1.61 90 1.01 14.2 0.85 1.19 95 0.82 14.2 1.10 0.74

Example 12 Extraction of Acetic Acid Using P_(666,14)-Isobutyrate and i-HOBu

Tie line data at both high (typically around 20 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 12.

Roughly equal masses of water and extraction solvent comprising different amounts of the carboxylate and NIO solvent were added to a glass vial. Isobutyric acid was tested as the NIO solvent. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 12.

TABLE 12 Effect of Isobutyric Acid on Acetic Acid Extraction by P_(666,14)-Isobutyrate Amount of Isobutyric HOAc at Acid P_(cont) Extraction (mole %) P_(cont) (wt %) R_(extr) Factor (ε) 10 3.19 19.3 0.273 11.69 30 2.08 20.6 0.255 8.16 50 1.67 18.8 0.275 6.08

Example 13 Extraction of Acetic Acid Using P_(666,14)-n-Octanoate and/or N-Octanoic Acid

Tie line data at both high (typically around 20 to 25 wt % of HOAc in the organic phase) and low acetic acid concentrations (typically around 0.2 to 5 wt % of HOAc in the organic phase) were measured at 20° C. for each solvent listed in Table 13.

Roughly equal masses of water and extraction solvent comprising different amounts of the carboxylate and NIO solvent were added to a glass vial. N-octanoic acid was tested as the NIO solvent. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by NMR for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to acetic acid weight ratio at high acid concentration, R_(extr). Results are given in Table 13.

TABLE 13 Effect of Octanoic Acid on Acetic Acid Extraction by P_(666,14)-Octanoate Amount of Octanoic HOAc at Acid P_(cont) Extraction (mole %) P_(cont) (wt %) R_(extr) Factor (ε) none 3.17 19.9 0.24 12.99 50 1.03 25.7 0.21 4.76

Example 14 Effect of Temperature on Extraction of Acetic Acid Using NIO Solvents

Tie line data were measured at 22 and 40° C. for each solvent listed in Table 14.

Roughly equal masses of water and solvent were added to a glass vial. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. Results are given in Table 14.

TABLE 14 The Effect of Temperature on Acetic Acid Partitioning in NIO Solvents HOAc at T P_(cont) Extraction Solvent (° C.) P_(cont) (wt %) R_(extr) Factor (ε) MTBE 22 0.86 2.0 0.46 1.88 MTBE 40 0.70 2.0 0.50 1.40 MIBK 22 0.67 3.9 1.14 0.59 MIBK 40 0.75 8.4 0.73 1.02 i-PrOAc 22 0.57 1.7 0.56 1.02 i-PrOAc 40 0.54 1.8 0.65 0.83

As seen from Table 14, the partition coefficient and extraction factor responds in different ways to changes in temperature in different NIO solvents.

Example 15 Effect of Temperature on Extraction of Acetic Acid Using P_(666,14)-Carboxylates

Tie line data were measured at 20 and 75° C. for each solvent listed in Table 15.

Roughly equal masses of water and solvent were added to a glass vial. Acetic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acetic acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and acetic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. Results are given in Table 15.

TABLE 15 Effect of Temperature on Extraction of Acetic Acid by P_(666,14)-Carboxlates HOAc Extrac- Carboxylate NIO T at P_(cont) tion Solvent Solvent (° C.) P_(cont) (wt %) R_(extr) Factor (ε) P_(666,14)-2EH none 20 3.24 20.4 0.21 15.21 P_(666,14)-2EH none 75 1.86 20.2 0.24 7.59 P_(666,14)-hexanoate none 20 3.32 20.3 0.25 13.48 P_(666,14)-hexanoate none 75 1.89 21.0 0.12 16.15 P_(666,14)-hexanoate 10 mol % 75 2.14 21.5 0.24 8.91 hexanoic acid P_(666,14)-2EB none 20 2.15 21.6 0.24 8.91 P_(666,14)-2EB none 75 1.87 20.9 0.27 6.94 P_(666,14)-2EB 10 mol % 75 2.24 20.0 0.26 8.63 2-EB acid

As with NIO solvents (see Table 14), the partition coefficients and extraction factors both increase and decrease with respect to temperature in different phosphonium carboxylate solvents (see Table 15). It is notable that, in some cases, the extraction factor increases with increasing temperature.

Procedure for Examples 16 Through 19

Tie line data were measured for the extraction of lower carboxylic acids (formic acid, propionic acid, n-butyric acid, and acrylic acid) from water using three NIO solvents MIBK, MTBE, and n-butyl acetate, and a carboxylate solvent P_(666,14)-2EH.

Tie line data at both high (typically around 10-25 wt % of carboxylic acid in the organic phase) and low acid concentrations (typically around 1 to 5 wt % of acid in the organic phase) were measured for each solvent at 40° C.

Roughly equal masses of water and solvent were added to a glass vial. The desired amount of carboxylic acid was added to the solvent-water mixture in amounts sufficient to yield either high or low acid concentration data. Once the acid was added, the mixture was agitated vigorously, and subsequently was allowed to separate into clear phases while maintaining the specified temperature. Each phase was sampled and analyzed by gas chromatography for water and carboxylic acid weight percent. These data were used to calculate partition coefficients, with the controlling partition coefficient, P_(cont), taken as the lesser of the partition coefficients at high and low acid concentrations. The data were also used to calculate the water to carboxylic acid weight ratio at high acid concentration, R_(extr).

Example 16 Extraction of Formic Acid with NIO Solvents Compared to P_(666,14)-2EH

This example presents extraction factors for the extraction of formic acid with NIO solvents MTBE, MIBK, and n-butyl acetate, and P_(666,14)-2EH following the extraction procedure outlined above. Results are presented in Table 16.

TABLE 16 Partitioning of Formic Acid in NIO Solvents and P_(666,14)-2EH (40° C.) HOAc at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) MTBE 0.7 16.8 0.69 1.0 MIBK 0.6 14.3 0.74 0.8 BuOAc 0.4 10.0 0.58 0.6 P_(666,14)-2EH 1.35 16.5 0.191 7.0

Example 17 Extraction of Propionic Acid with NIO Solvents Compared to P_(666,14)-2EH

This example presents extraction factors for the extraction of propionic acid with NIO solvents MTBE, MIBK, and n-butyl acetate, and P_(666,14)-2EH following the extraction procedure outlined above. Results are presented in Table 17.

TABLE 17 Partitioning of Propionic Acid in NIO Solvents and P_(666,14)-2EH (40° C.) Propionic Acid at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) MTBE 3.2 20.2 0.30 10.7 MIBK 2.6 18.8 0.46 5.5 BuOAc 2.0 17.7 0.30 6.50 P_(666,14)-2EH 7.64 25.1 0.16 47.3

Example 18 Extraction of n-Butyric Acid with NIO Solvents Compared to P_(666,14)-2EH

This example presents extraction factors for the extraction of n-butyric acid with NIO solvents MTBE, MIBK, and n-butyl acetate, and P_(666,14)-2EH following the extraction procedure outlined above. Results are presented in Table 18.

TABLE 18 Partitioning of n-Butyric Acid in NIO Solvents and P_(666,14)-2EH (40° C.) Butyric Acid at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) MTBE 9.9 25.2 0.20 48.5 MIBK 8.4 23.0 0.31 27.3 BuOAc 6.8 22.1 0.22 30.9 P_(666,14)-2EH 4.6 22.0 0.17 27.0

Example 19 Extraction of Acrylic Acid with NIO Solvents Compared to P_(666,14)-2EH

This example presents extraction factors for the extraction of acrylic acid with NIO solvents MTBE, MIBK, and n-butyl acetate, and P_(666,14)-2EH following the extraction procedure outlined above. Results are presented in Table 19.

TABLE 19 Partitioning of Acrylic Acid in NIO Solvents and P_(666,14)-2EH (40° C.) Acrylic Acid at P_(cont) Extraction Solvent P_(cont) (wt %) R_(extr) Factor (ε) MTBE 3.6 24.2 0.31 11.8 MIBK 2.7 21.6 1.40 6.8 BuOAc 2.1 20.6 0.30 6.9 P_(666,14)-2EH 4.8 20.9 0.17 28.5

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 

We claim:
 1. A solvent for extracting a C₁ to C₄ carboxylic acid from water, the solvent comprising: (a) a quaternary phosphonium carboxylate salt; and (b) a non-ionic organic solvent, wherein the non-ionic organic solvent is not the extract, and wherein the carboxylate salt has the general formula 1:

wherein R¹, R², R³ and R⁴ are each independently a C₁ to C₂₆ hydrocarbyl group, provided that R¹, R², R³, and R⁴ collectively have a total of at least 24 carbon atoms; and R⁵ is hydrogen or a C₁ to C₂₄ non-aromatic hydrocarbyl group.
 2. The extraction solvent according to claim 1, wherein the carboxylate salt comprises a tetraalkylphosphonium salt of formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acids, n-octanoic, 2-ethylhexanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, palmitic acid, stearic acid, oleic acid, linolenic acid, mixed vegetable-derived acids, or chloroacetic acid.
 3. The extraction solvent according to claim 1, wherein the carboxylate salt comprises a trihexyl(tetradecyl)phosphonium salt of formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic, 2-ethylhexanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, palmitic acid, stearic acid, oleic acid, linolenic acid, mixed vegetable-derived acids, or chloroacetic acid.
 4. The extraction solvent according to claim 1, wherein the carboxylate salt comprises trihexyl(tetradecyl)phosphonium carboxylate.
 5. The extraction solvent according to claim 1, which comprises at least two of the carboxylate salts.
 6. The extraction solvent according to claim 1, wherein the non-ionic organic solvent is selected from the group consisting of higher carboxylic acids, ethers, esters, ketones, aromatic hydrocarbons, chlorinated hydrocarbons, and nitriles.
 7. The extraction solvent according to claim 6, wherein the non-ionic organic solvent comprises a higher carboxylic acid.
 8. The extraction solvent according to claim 7, wherein the higher carboxylic acid is selected from the group consisting of n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic, 2-ethylhexanoic acids, nonanoic acids, decanoic acids, dodecanoic acids, stearic acid, oleic acid, linolenic acid, and mixed vegetable-derived acids.
 9. The extraction solvent according to claim 8, wherein the higher carboxylic acid is selected from the group consisting of n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic acid, and 2-ethylhexanoic acid.
 10. The extraction solvent according to claim 1, wherein the non-ionic organic solvent is selected from the group consisting of esters containing four to six carbon atoms, ketones containing five to nine carbon atoms, and ethers containing four to eight carbon atoms.
 11. The extraction solvent according to claim 10, wherein the non-ionic organic solvent is selected from the group consisting of ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, i-propyl propionate, 2-pentanone, 3-pentanone, methyl isobutyl ketone, 3-methyl-2-butanone, 2-hexanone, 2-heptanone, cyclohexanone, 4-methyl-2-pentanone, 2,4-dimethyl-3-pentanone, 5-methyl-2-hexanone, 4-heptanone, 2-octanone, 5-nonanone, 2,8-dimethyl-4-heptanone, 3,3,5-trimethyl cyclohexanone, isophorone, diethyl ether, methyl propyl ether, dipropyl ether, di-isopropyl ether, methyl t-butyl ether, tertiary amyl methyl ether, ethyl butyl ether, toluene, m-xylene, p-xylene, and o-xylene.
 12. The extraction solvent according to claim 11, wherein the non-ionic organic solvent is selected from the group consisting of methyl isobutyl ketone, toluene, isopropyl acetate, and methyl t-butyl ether.
 13. The extraction solvent according to claim 1, which comprises at least two of the non-ionic organic solvents.
 14. The extraction solvent according to claim 1, which comprises 10 to 90 weight percent of the carboxylate salt and 10 to 90 weight percent of the non-ionic organic solvent.
 15. The extraction solvent according to claim 1, which comprises 50 to 90 weight percent of the carboxylate salt and 10 to 50 weight percent of the non-ionic organic solvent.
 16. A process for separating a C₁ to C₄ carboxylic acid from water, the process comprising: contacting a feed mixture comprising a C₁ to C₄ carboxylic acid and water with an extraction solvent comprising a quaternary phosphonium carboxylate salt and a non-ionic organic solvent at conditions effective to form (a) an extract mixture comprising the carboxylate salt, the non-ionic organic solvent, and at least a portion of the C₁ to C₄ carboxylic acid from the feed mixture and (b) a raffinate mixture comprising water and less of the C₁ to C₄ carboxylic acid compared to the feed mixture, wherein the non-ionic organic solvent is not the extract, and wherein the carboxylate salt has the general formula 1:

wherein R¹, R², R³ and R⁴ are each independently a C₁ to C₂₆ hydrocarbyl group, provided that R¹, R², R³, and R⁴ collectively have a total of at least 24 carbon atoms; and R⁵ is a C₁ to C₂₀ non-aromatic hydrocarbyl group.
 17. The process according to claim 16, wherein the carboxylate salt comprises a tetraalkylphosphonium salt of formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acids, n-octanoic, 2-ethylhexanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, palmitic acid, stearic acid, oleic acid, linolenic acid, mixed vegetable-derived acids, or chloroacetic acid.
 18. The process according to claim 16, wherein the carboxylate salt comprises trihexyl(tetradecyl)phosphonium carboxylate.
 19. The process according to claim 16, wherein the extraction solvent comprises at least two of the carboxylate salts.
 20. The process according to claim 16, wherein the non-ionic organic solvent is selected from the group consisting of higher carboxylic acids, ethers, esters, ketones, aromatic hydrocarbons, chlorinated hydrocarbons, and nitriles.
 21. The process according to claim 20, wherein the higher carboxylic acid is selected from the group consisting of n-butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, n-hexanoic acid, 2-ethylbutyric acid, heptanoic acid, n-octanoic, 2-ethylhexanoic acids, nonanoic acids, decanoic acids, dodecanoic acids, stearic acid, oleic acid, linolenic acid, and mixed vegetable-derived acids.
 22. The process according to claim 20, wherein the non-ionic organic solvent is selected from the group consisting of ethyl acetate, n-propyl acetate, n-propyl formate, i-propyl acetate, i-propyl formate, n-butyl acetate, n-butyl formate, i-butyl acetate, i-butyl formate, n-propyl propionate, i-propyl propionate, 2-pentanone, 3-pentanone, methyl isobutyl ketone, 3-methyl-2-butanone, 2-hexanone, 2-heptanone, cyclohexanone, 4-methyl-2-pentanone, 2,4-dimethyl-3-pentanone, 5-methyl-2-hexanone, 4-heptanone, 2-octanone, 5-nonanone, 2,8-dimethyl-4-heptanone, 3,3,5-trimethyl cyclohexanone, isophorone, diethyl ether, methyl propyl ether, dipropyl ether, di-isopropyl ether, methyl t-butyl ether, tertiary amyl methyl ether, ethyl butyl ether, toluene, m-xylene, p-xylene, and o-xylene.
 23. The process according to claim 16, wherein the extraction solvent comprises at least two of the non-ionic organic solvents.
 24. The process according to claim 16, wherein the extraction solvent comprises 10 to 90 weight percent of the carboxylate salt and 10 to 90 weight percent of the non-ionic organic solvent.
 25. The process according to claim 16, wherein the C₁ to C₄ carboxylic acid comprises acetic acid.
 26. The process according to claim 16, wherein the feed mixture comprises at least two of the C₁ to C₄ carboxylic acids.
 27. The process according to claim 16, wherein the feed mixture comprises 0.5 to 60 weight percent of the C₁ to C₄ carboxylic acid.
 28. The process according to claim 16, wherein the feed mixture is derived from the production of cellulose esters.
 29. The process according to claim 16, wherein the weight ratio of the extraction solvent to the feed mixture ranges from 0.2 to 10:1.
 30. The process according to claim 16, which further comprises: separating the extract mixture from the raffinate mixture; and recovering the C₁ to C₄ carboxylic acid from the extract mixture by distillation at atmospheric pressure or lower. 